As has been said, number of users isn't the key thing with a forum. Number of posts and active users at any one time are more an issue.

If thats 10k active, then you'll need some serious hosting. I run a couple of vBulletin forums for various things, one of them is on a form of shared hosting, another is on a VPS (and that handles up to 2.5k active).

Certainly, you can with access to a server (i.e. not a shared host) run a lot more tools to support and improve performance making the forum go a long way.

Jelsoft (vBulletin's makers) recommended we had a 3-4 server setup for our load, we've never needed it yet.

Is it a good idea to get a cheap hosting package now (like fatcow) and move to a better one as my users scale up? Or just get a robust one now?

I'm assuming it's a new forum - and the answer is yes, definitely get a cheap hosting plan to start with. You are very likely to not end up using all the resources you think you'll use, or growing more slowly than you think you will.

For instance, even if you have 200 users online, many of them will be reading or not actually accessing the site all at once. Let's say that you have 100 users online over a period of 10 minutes, just as an example. If each of those users looks at 10 pages in that 10 minutes, you have 1 page per minute per user, or roughly 1.5 page hits per second. You can also reduce load on the server by using something like cloudflare to cache pages so the server is not hit as often. (I'm afraid I don't know how well cloudflare works with your forum software, best to ask them).

I didn't launch my forum yet. It's a unique niche with almost nil competition. Someone tried to start a forum in this niche some 2yrs ago and he had 4000 members within 3 months. His site and forums were self built and pathetic quality. No look and feel at all. He was trying to monetize too quickly or I don't know what happened. The domain is now available for registration!

I am trying to do it more professionally ths time. Planning to get vbulletin - keen on creating very good user experience.

So my projections are :5000 members in 6 months...not sure how to estimate the number of people online Its likely to be a very active forum once it catches attention.

Do I start with a shared host ? If yes, at what point should I be moving to vps? What point to a dedicated host?

I didn't launch my forum yet. It's a unique niche with almost nil competition. Someone tried to start a forum in this niche some 2yrs ago and he had 4000 members within 3 months. His site and forums were self built and pathetic quality. No look and feel at all. He was trying to monetize too quickly or I don't know what happened. The domain is now available for registration!

I am trying to do it more professionally ths time. Planning to get vbulletin - keen on creating very good user experience.

So my projections are :5000 members in 6 months...not sure how to estimate the number of people online Its likely to be a very active forum once it catches attention.

Do I start with a shared host ? If yes, at what point should I be moving to vps? What point to a dedicated host?

Will you monitize this site throughout, what have you estimated in your business plan for the amount of money you will make. It makes a difference if you are generating income or not.

Shared hosts try to maximize the number of sites on a single server; therefore if one of your goals is to create a good user experience (professionally) then shared hosting is not good for forum software, a shared host will become overwhelmed very quickly using vbulletin. If you can find a reasonably priced VPS then start there.

If you are in a market where you can pay for what bandwidth/storage is used then you may consider Cloud hosting like Amazon S3 or EC2, however for most non-static sites that generate a lot of traffic but little income, cloud hosting can get very costly.

You will work with your host regarding your resource consumption and will consult with them when you know that you are hitting your VPS resources too hard.

It is easiest if you choose a host that has a good path/pricing from shared -> VPS -> dedicated, so choose your hosting carefully. If you haven't reviewed the free "Find a Great Hosts in 5 Steps' document that SitePoint has, I'd recommend you look through this.

I suggest you go with godaddy,... it's cheap for your requirements... or check the iweb.com for better solution.

If you've checked the threads here over 2012 there are a number of people that complained regarding godaddy hosting, in fact most people say their a good registrar but not a good host. I personally don't have hosting experience with them, but all I'm suggesting is that you take the time to properly research your host so you know what issues others have dealt with and you don't get caught by their policies or pricing tricks.

I don't plan to monetise until I hit 10k members.....that's atleast a year or two away!

Start with a shared platform but ideally not Endurance. Choose someone that is more focussed on customer service than just the numbers. Then upgrade as you grow; most decent hosts will assist you in doing this and let you know when its time to upgrade.

Also, you could do very simple monetization right from the start with something like Skimlinks. That isn't too intrusive but could yield revenues sufficient to cover your hosting costs.

You could start with a share hosting account if your number of active users at any time is low. However, if 10,000 users are online all at once (which I doubt) you should be using a dedicated server. I would recommend talking a look at wiredtree.com